Add feature flag for summarize code review model upgrade

What does this MR do and why?

• This MR adds a new feature flag, summarize_code_review_claude_3_7_sonnet, which will be used to rollout the model upgrade to Claude 3.7 Sonnet for summarize code review feature.
• MR that adds a new prompt for Claude 3.7: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!2066 (merged)

How to set up and validate locally

• Start reviewing a MR and add a bunch of comments
• Click on Finish review and make sure Claude 3.5 model is used by checking the logs in gdk tail -f gitlab-ai-gateway | grep "Request to LLM"
• Enable the feature flag and make sure Claude 3.7 model is used by checking the logs in gdk tail -f gitlab-ai-gateway | grep "Request to LLM"

Screenshots

• Claude 3.7 model is used when feature flag is enabled:

• Claude 3.5 model is used when feature flag is disabled:

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #521383 (closed)